The present invention pertains to subsea containers and more particularly to subsea containers that have no hard wire connection to surface equipment, and are used to house equipment that is deployed, performs a function such as data collection and is retrieved to withdraw the acquired data at a subsequent time.
Presently there are several types of containers in which data gathering equipment, such as seismic data detection systems, are housed for subsea application. These containers are called ocean bottom seismometers, and have a pressurized housing with an acoustic pulse detector, power supply and recorder within.
Problems may be present in several areas when subsea containers are used. First, obtaining the data from the Ocean Bottom Seismometer (OBS) required that the unit be opened to retrieve the data. Data is usually retained by a tape recorder and the tape must be physically removed to allow playback for further processing. A door is provided with seals to protect the internal equipment from the corrosive effects of seawater and to prevent shorting of various electrical components. Opening the compartment door frequently, that is, each time the OBS is used, causes wear on the seals provided and can result in seal failure when exposed to the high pressures encountered in subsea use. Replacement of the door seals is a possible solution but it requires additional man hours in assuring proper placement of the seals and waste in replacing seals that would not fail.
Maintenance of Ocean Bottom Seismometer units adds another complication to the problem of door seals. Not only must the door be opened to physically remove the tapes from a tape recorder, but the door must also be opened to monitor the power supply. A great deal of sophisticated electronic equipment along with the tape recorder are contained in an Ocean Bottom Seismometer. As such a power supply normally a battery pack, must be provided. Normally, the power supply is located in the compartment with the recording unit to provide power for the seismic data acquisition system.
A second problem that occurs in the deployment and retrieval of Ocean Bottom Seismometers is one that is environmental related. When an OBS unit is used in an area having a muddy, almost a slurry, bottom, an OBS unit may sink into the bottom and retrieval may be hampered by the suction force exerted by the mud. An Ocean Bottom Seismometer, being used to collect seismic data, must be close coupled to the earth. Since it is being used at varying depths below the surface of the ocean it must weigh more than the water which its volume displaces. Furthermore, it must have a significant subsurface weight to resist the forces of ocean bottom currents. A slurry mud bottom does not provide a hard surface upon which the Ocean Bottom Seismometer rests. As a result, an OBS unit sinks into the mud to a point of density equilibrium, where the density of the medium below the Ocean Bottom Seismometer is greater than that of the unit itself. When an OBS unit is to be retrieved, the thick mud slurry may exert a suction force hindering its return to the ocean surface.
In prior art, a simple release of ballast has been used to increase the Ocean Bottom Seismometer's bouyancy to return to the surface. These methods have not always been successful, not only because of mud problems but also due to possible release mechanism failure.
In subsea seismic exploration, three general types of seismic data aquisition systems are presently in use. The first, most widely used, is to tow a line of acoustic pulse generators between a towing vessel and a line of acoustic pulse detectors, such as hydrophones. The acoustic pulse generators produce acoustic pulses which are reflected and refracted by subsurface formations within the earth's crust. Normally, the line of acoustic pulse detectors respond only to reflected waves since refracted waves emerge from the formation and return to the surface many miles from their point of incidence. The second and third types of data acquisition systems are used to detect refracted waves. These are an Ocean Bottom Cable (OBC) and an Ocean Bottom Seismometer (OBS).
An Ocean Bottom Cable is similar to the line of seismic pulse detectors that is towed behind a marine vessel to detect reflected seismic acoustic waves. This line of seismic pulse detectors is deployed several miles away from a marine vessel towing acoustic pulse generators and placed on the ocean bottom. The acoustic pulse detectors are close coupled to the earth surface and are used to detect refracted waves when the OBC is placed remote from the acoustic pulse generators. When the Ocean Bottom Cable is used in close proximity to the acoustic pulse generators, reflected seimic acoustic pulse are detected.
Similarly, Ocean Bottom Seismometers are used to detect and record refracted seismic acoustic pulses. An OBS unit is a self contained unit having seismic pulse detectors and tape recorders. It is deployed and placed on the ocean bottom. At a predetermined time, it begins recording refracted seismic data and continues until its recording capability has been completed. The ocean bottom seismometer receives a signal, ballast is released and the OBS unit returns to the ocean surface, where it is retrieved. Once retrieved, the data is removed from the Ocean Bottom Seismometer for further processing.
The collection of refraction data may be done by either the Ocean Bottom Cable or the Ocean Bottom Seismometer, each having applications which are impractical for the other.